1. Field of the Invention
The invention relates to the field of providing fluids downhole into a subterranean formation which are mixed for the first time within the subterranean formation, and in particular, to methods of fracturing employing coiled tubing. Methods are provided for separately administering fracturing fluid components which are mixed for the first time at a point downhole, allowing for combination of said components at a later time and at a location which is adjacent to the subterranean formation to be fractured.
2. Description of the Prior Art
In the recovery of oil and gas from subterranean formations it is common practice to fracture the hydrocarbon-bearing formation, providing flow channels for oil and gas. These flow channels facilitate movement of the hydrocarbons to the wellbore so they may be produced from the well. Without fracturing, many wells would cease to be economically viable.
In such fracturing operations, a fracturing fluid is hydraulically injected down a wellbore penetrating the subterranean formation. The fluid is forced down the interior of the wellbore casing, through perforations, and into the formation strata by pressure. The formation strata or rock is forced to crack open, and a proppant carried by the fluid into the crack is then deposited by movement of the viscous fluid containing proppant into the crack in the rock. The resulting fracture, with proppant in place to hold open the crack, provides improved flow of the recoverable fluid, i.e., oil, gas, or water, into the wellbore.
Fracturing fluids customarily comprise a thickened or gelled aqueous solution which has suspended therein proppant particles that are substantially insoluble in the fluids of the formation. Proppant particles carried by the fracturing fluid remain in the fracture created, thus propping open the fracture when the fracturing pressure is released and the well is placed on production. Suitable proppant materials include sand, walnut shells, sintered bauxite, or similar materials. The propped fracture provides a larger flow channel to the well bore through which an increased quantity of hydrocarbons can flow, thereby increasing the production rate of a well.
Hydraulic fracturing fluids usually contain a hydratable polymer which is crosslinked (and therefore thickened) on the surface of the ground by mixing it with crosslinking agent. The crosslinking agent thickens the fracturing fluid prior to and during the pumping of the fluid downhole. The polymer typically is hydrated upon the surface of the ground in a batch mix operation for several hours in a chemical mixing tank, and then mixed with a crosslinking agent over a period of time to greatly thicken the fluid and increase its viscosity so that is can carry the proppant into the fracture. The fluid is transformed by crosslinking from a water-like consistency into a thick fluid having a viscous jello-like consistency.
One difficulty with such processes is that a large number of additives are required to function at high temperatures, elevated pressures, and after undergoing significant frictional shear forces. These additives include, for example: bactericides, antifoam agents, surfactants to aid dispersion, pH control agents, chemical breakers, enzymatic breakers, iron control agents, fluid stabilizers, crosslinkers, crosslinking delay additives, antioxidants, salt(s) and the like. These additives must be formulated correctly (which is a difficult task), transported to location, mixed, pumped and metered accurately to execute the fracturing job properly. There are several disadvantages and costly problems associated with preparing and using polysaccharides which are pre-mixed with crosslinking agents on the surface of the ground and then passed downhole for later use as viscosifying proppant carrying compounds in the formation.
In fracturing, it would be ideal to achieve crosslinking of the fluid at a time just before the fluid reaches the perforation so that the fluid carries the proppant properly through the perforations and over the length of the fracture. If the crosslinking takes place after the fluid reaches the perforation, then a risk is presented that the proppant will not be carried across the perforations or that the fluid will not perform in the fracture. In either case, the fracturing event will not provide the anticipated results. On the other hand if crosslinking is taking place too early as the fluid makes its way down the wellbore, significant friction losses will be generated, increasing the pressure on surface and making execution of the job more difficult. Further, the fluid may be irreversibly degraded by the high level of shear in the wellbore, which in some extreme cases can jeopardize the entire job, such as in high temperature deep wells in which the fluid travels a long distance for a long time.
Achieving perfect timing for crosslinking is made even more difficult by the fact that every well has its own characteristics of depth, temperature and pump rate. Thus, any attempts to predetermine fluid crosslink timing at the surface requires a different formulation for every well. This sort of customization of fracturing methods is expensive and unmanageable. The problem is compounded when the conditions of treatment are extreme in terms of well depth and temperature. In some cases it can become the limiting factor in the execution of the job. Another limitation and difficulty with the conventional mode of fracturing is the delay between the time when the operator decides to change the viscosity of the fluid and the time when the change actually is implemented downhole. The change in fluid properties can be obtained by changing the composition on surface. But when such surface adjustment is employed, it then takes several minutes for the fluid with the modified composition to travel downhole to the point at which the change is required. A screen-out, in which proppant falls out of solution, blocking fluid flow and raising pressure to extremely high levels, can occur in a matter of seconds if conditions are not correct. This time delay in achieving fluid change reduces significantly the flexibility of the fracturing operation in terms of reacting to unforeseen events.
Fluids described above in the prior art, and used in the industry, are designed with compositions having pre-determined properties that are averaged in an attempt to apply the fluids successfully to a wide variety of wellbore temperatures, pressures, and other characteristics. The more a particular wellbore deviates from the average, the less successful the particular composition or procedure will be in fracturing the well with maximum efficiency.
It has been known in the art to provide gaseous substances through a coiled tubing, thereby generating a foamed fracturing fluid downhole, for certain applications. These gaseous substances include, for example, nitrogen or carbon dioxide. Unfortunately, however, the limitations and problems previously described above often apply equally as well to such foamed fracturing fluids. In such instances, the crosslinking still occurs early, prior to or concurrently with the pumping of the fluid downhole, and polysaccharides usually have been mixed with crosslinkers and other substances above the ground, and then pumped downhole together as a mixture.
What is needed in the industry is a method of fracturing a wellbore in which the timing and degree of crosslinking is optimized and the adverse effects of shear degradation of the fluid are minimized. A method of fracturing using a fluid that is not made highly viscous prior to or immediately upon beginning its travel down the wellbore is desirable. Such a method of fracturing could reduce the friction pressures which must be applied to the fluid to transfer it downhole, thereby improving the fluid performance and reducing equipment horsepower requirements.
A desirable process of fracturing is shown where the viscosity of the fluid is not predetermined upon the above-ground mixing of polymer, crosslinker, activator, and breakers; but instead, viscosity of the fluid is adjustable after the initiation of fracturing and/or pumping. A method of customizing in real time the rheology characteristics of the fracturing fluid as the fluid is being applied to the subterranean formation to meet particular wellbore or reservoir characteristics is highly desirable.